Steam reforming of a hydrocarbon to manufacture syngas is a very well known process. One popular technique is to use an autothermal reformer in conjunction with a reforming exchanger, as described in U.S. Pat. No. 5,011,625 to Le Blanc, which is hereby incorporated by reference herein in its entirety. Briefly, the hydrocarbon and an oxygen source are supplied to the autothermal reformer. The combustion reaction is exothermic and supplies the heat needed for the catalytic reforming reaction that occurs in the autothermal reformer, which is endothermic, to produce a relatively hot reformed gas. The hot gas from the autothermal reformer is then used as a heat source in the reforming exchanger, which is operated as an endothermic catalytic steam reforming zone. In the reforming exchanger, a feed comprising a mixture of steam and hydrocarbon is passed through catalyst-filled tubes. The outlet ends of the tubes discharge the endothermically reformed gas near the shell side inlet where it mixes with the hot gas from the autothermal reformer. The hot gas mixture is then passed countercurrently across the tubes in indirect heat exchange to supply the heat necessary for the endothermic reforming reaction to occur.
Reforming exchangers are in use commercially and are available, for example, from Kellogg Brown & Root, Inc. under the trade designation KRES. Various improvements to the reforming exchanger design have been made, as disclosed in, for example, U.S. Pat. No. 5,362,454 to Cizmar et al., which is hereby incorporated by reference herein in its entirety.
The present invention addresses improvements to the basic reforming exchanger design. The primary design consideration is to minimize the capital cost of the equipment, which is complicated because expensive alloys must be used to construct the tube bundle and tube sheets for the relatively high operating temperatures and pressures. Another design consideration is to maximize the capacity of the reforming exchanger within the practical limits of fabrication capabilities. It is also desirable to minimize the size and weight of the reforming exchanger to facilitate maintenance operations that require removal of the tube bundle.
Our approach to reducing the capital cost and increasing the capacity of the reforming exchanger is to increase the heat transfer rate by increasing the surface area available for heat transfer. Increasing the length of the conventional catalyst-filled tubes in the existing reforming exchanger design, however, was not practical because the tube side pressure drop (ΔP) would increase beyond that permitted without unduly complicating the tube sheet and tube sheet support designs, as well as increasing upstream operating pressures and compression costs. Furthermore, longer tubes would complicate the maintenance operations involving removal of the tube bundle.
The other approach to increasing the heat transfer area is to reduce the diameter of the catalyst-filled tubes. However, it was a commonly held belief among the reforming reactor designers that the inside diameter of the heat transfer tubes had to be a minimum of 5 times the diameter or other largest edge dimension of the catalyst particles, supposedly because of packing, bridging, flow channeling, and other potential problems. For example, James T. Richardson, Principles of Catalyst Development, Plenum Press, New York, N.Y., p. 8 (1986) (citing E. G. Christoffel, “Laboratory Reactors and Heterogeneous Catalytic Processes,” Catal. Rev.—Sci. Eng., vol. 24, p. 159 (1982)), reports that the reactor to particle diameter ratio should be from five to ten, with the reactor length at least 50-100 times the particle diameter, to ensure that the flow is turbulent, uniform, and approximately plug flow.
To observe these design criteria would mean that reducing the tube diameter and increasing the number of tubes, as a means of increasing the available surface area, would require using a smaller catalyst structure. For example, in tubes having a 2-in. inside diameter (ID), the longitudinally bored, cylindrical catalyst shapes, also known as Raschig rings, prevalent in reforming exchangers used in the art would typically measure 0.315-in. outside diameter (OD) by 0.125-in. ID by 0.31-in. long. When small-ID tubes were specified, it was thought that the size of the catalyst particles had to be correspondingly reduced to adhere to the traditional equation Dt/Dp>5, wherein Dt is the inside diameter of the tubes in the reforming exchanger and Dp is the maximum edge dimension of the catalyst structure. Unfortunately, the use of smaller catalyst particles in smaller tubes, to observe this conventional design criterion, resulted in an unacceptable increase in tube side pressure drop. Conversely, because existing reforming exchanger designs were already at or near the maximum ratio of catalyst size to tube ID, the catalyst size could not be increased in the existing tube design as a means for reducing the pressure drop per unit of tube length so as to allow the use of longer tubes. It appeared as if there would be no practical way to increase the heat transfer, and that the ultimate capacity limits of the reforming exchanger design had been reached.